This invention relates to a nonfibrous, unoriented piezoelectric copolymer resin sheet having improved hydrophonic properties which properties are thermally stable. More particularly, it relates to an unoriented, unstretched, nonfibrous, electrically polarized sheet of a copolymer resin of about 65 up to 85 mol percent of vinylidene fluoride and 15 to about 35 mol percent of at least one copolymerizable substituted monolefinic monomer, said sheet having a combination of piezoelectric properties making it highly desirable in hydrophonic applications. In addition, this invention relates to the process of preparing said sheet wherein it is heated at high temperatures for a prolonged time period to improve its dielectric breakdown strength and piezoelectric properties which are imparted when the sheet is poled.